1. Field of the Invention
This invention relates generally to a window glass antenna system for receiving radiowaves of AM (Amplitude Modulation) and FM (Frequency Modulation) bands and, in particular, to a window glass antenna system in which a noise blocking circuit is provided for reducing noises from an antenna of the system.
2. Description of the Related Art
An example window glass antenna system is disclosed in Japanese Utility Model Kokoku (Post-Exam) Publication No. HEI 7-22892 and U.S. Pat. No. 5,239,302 of the present assignee. This prior window antenna system includes an impedance matching circuit provided between a window glass antenna for receiving AM band radiowaves and a feeder cable connected to a radio receiver. The impedance matching circuit may be caused to undergo series resonance and parallel resonance by frequencies of AM band radiowaves received by the window glass antenna to thereby increase the reception level of AM radio reception signals fed to the radio receiver.
Reference is made to FIG. 12 hereof which shows the general arrangement of the known window glass antenna system.
As shown in FIG. 12, the window glass antenna system 50 has upper and lower heater patterns 52 and bus bars 53, 54, 55, which are deposited on an area of a rear window glass (pane) 51 where defogging is required. The upper and lower heater patterns 52 are all connected at one end through the bus bar 55 and fed with power via the bus bars 53, 54 at opposite ends thereof.
Heating of the upper and lower heater patterns is achieved by supplying an electrical current from a power source +B through a wire harness 62W, through a choke coil 61A, through a feeder 62, through the bus bar 53, through the lower heater pattern 52, through the bus bar 55, through the upper heater pattern 52, through the bus bar 54, through a feeder 63 and through a choke coil 61B and then to ground.
Choke coils 61A, 61B are set to have a high impedance relative to the AM band frequencies as viewed at the power source +B from the bus bars 53, 54 so that the heater patterns 52 can be used as antennas. The choke coils 61A, 61B are also set to have a high impedance as viewed at the bus bars 53, 54 from the power source +B so that AM radio frequency band noises arising from the power source side can be reduced.
Decoupling capacitor 64 is provided to block power source noises to be induced from the power source +B into the heater patterns 52 forming an AM antenna.
AM band radio reception signals are supplied via an output 56 of the bus bar 55 to an impedance matching circuit 65. The matching circuit 65 is composed of inductors and a resistor and has an inductance value which is selected so that impedance as viewed from an input terminal 66A of the radio receiver (not shown) causes series resonance and parallel resonance in AM band frequencies to thereby increase the level of the AM band radio reception signals of the input terminal 66A. Designated by reference numerals 65A, 65B are input and output terminals of the matching circuit 65.
Shown in FIG. 13 is an equivalent circuit diagram of the known window glass antenna system. In this figure, reference character Eo represents an induced voltage of AM radio frequency band generated in the heater patterns (antenna) 52. Reference characters R.sub.A, C.sub.A and C.sub.B respectively represent equivalent resistance, equivalent capacitance and stray capacitance of the heater patterns (antenna) 52. Designated by reference character C.sub.L is stray capacitance of feeders 62, 63. R.sub.L, C.sub.D and L.sub.X represent equivalent resistance, equivalent capacitance and equivalent inductance of the choke coils 61A, 61B, respectively. C.sub.K designates stray capacitance of a feeder cable 57.
Matching circuit 65 is composed of an inductor L.sub.A, inductor L.sub.B and a resistor R. The inductor L.sub.A is connected in series with the antenna stray capacitance (combined capacitance) as viewed at the antenna from the radio receiver input 66A whilst the inductor L.sub.B is connected in parallel with the antenna stray capacitance (combined capacitance) as viewed at the antenna from the radio receiver input 66A. Thus, in the AM radio frequency band, inductor L.sub.A and stray capacitance (combined capacitance) jointly cause series resonance while inductor L.sub.B and stray capacitance (combined capacitance) jointly cause parallel resonance. As a result, the AM radio frequency band reception signals received by the antenna can be increased in level and supplied to the radio receiver, whereby improvement in the reception sensitivity relative to the AM radio frequency band is achieved.
However, in the known window glass antenna system 50, when noises are induced into the wire harness 62W connecting the power source +B and choke coil 61A, the noises may not be fully attenuated in the choke coil 61A and may thus be partially fed into the heater patterns 52 forming the AM antenna. As a result, the noises are eventually received by the radio receiver through the matching circuit 65, thus presenting the AM broadcast with such noises and leading to the deterioration of the quality of the resulting antenna system.
When noises of the order of 100-200 kHz are induced into the wire harness 62W from various electrical parts such as a stop (brake) light and a direction indicator (blinker), or from an alternator, noise components are generated in the AM broadcast frequency band of 522-1620 kHz through cross modulation within the radio, thus deteriorating the SIN (signal-to-noise) ratio.
One may propose the approach to establish high impedance relative to noise frequencies by increasing inductance of the choke coils 61A, 61B to thereby reduce noises to be induced into the AM antenna to such an extent that they pose no problems in practical uses. However, this results in large-sizing of the choke coils and hence is costly and impractical.